Interfacial catalysts

Lipases are unusual hydrolytic enzymes because they act on substrates providing an interface (with few exceptions). This feature has been historically used to distinguish lipases from esterases, the latter of which act on substrates in true solution (Jensen, 1983). The distinction of lipases as interfacial catalysts can make kinetic characterization a challenge, because relevant substrate concentrations are expressed in terms of area and not concentration. 

A pol3nner-Cu complex coated electrode was successfully applied as the interfacial catalyst for the oxidative polymerization of phenol. 

These results suggest some factors for the preparation of electrodes with electron-transfer ability, (i) There is an optimum thickness of the coated polymer film for the electron-transfer reaction, (ii) The polymer matrix should be flexible. Otherwise, the matrix retards the diffusion of a counter ion and suppresses the effective collision between redox sites. (iii) A hydrophilic but uncharged polymer domain is suitable for the mass-transfer process in catalysis. A series of polymer complex coated electrodes were prepared as interfacial catalysts (16), one example is given below. 

Yang [234] has developed a theoretical model to investigate the effects of mass transfer and distribution of the catalyst within the third liquid phase and organic or aqueous phase on the overall reaction rate. The modeling considers the dispersed organic droplet surrounded by an interfacial catalyst layer under agitation conditions, as shown in Fig. 10. This type of droplet is similar to some oil/water emulsions in the presence of surfactants. The reactant... 

The interaction of mono- and dihalo benzenes with alkoxy ions is accelerated by the presence of polyethylene glycols which act as interfacial catalysts [182]. The percentage of conversion ( ) of CI2C6H4 in the absence of catalysts but in the presence of PEG with the weight-average molecular weight (A/w) of 150,1000,6000, 8000 and 2-10 and at 140-150°C and 6h reaction time, is 7.1, 8.2, 20.1, 33.0 and 28.1%, respectively. Bromobenzenes are more reactive than dichlorobenzenes. In the presence of PEG with Mif/ = 6000, the conversion values are 71.5 and 33.0%, respectively. The effectiveness of alkoxylation depends on the alcohol type and decreases in the sequence primary > secondary > tertiary. The values for 0-CI2C5H4 esteriflcation of methyl, isopropyl and rert-butyl alcohols, in the presence of a catalyst with Mvv = 8000 at 150°C and 6h reaction time are 66.0, 13.5, and 5.1%, respectively. 

Furthermore, the hydrolysis of butyl acetate and methyl pivalate in benzene in the presence of KOH at 25 °C as well as the reaction of potassium phenolate with benzyl chloride in boiling acetonitrile are accelerated by addition of polyoxyethylene [183]. The catalytic effect of POE is augmented by an increase in the number of oxyethylene units, i.e. 1 <6< 12. PEO is also an interfacial catalyst of the reaction of phenol and 2,4,6-trimethylphenol with methyl iodide in water-chloroform and dichloromethane. The kinetic study of the reaction between benzyl chloride and potassium acetate in the presence of PEO of variable molecular weight in toluene and butanol has been performed with IR spectroscopy [184]. The dissolution of a reagent of poor solubility is apparently a rate-limiting step of the reaction in a solution of low polarity (toluene). The presence of PEO impurities in toluene has been detected. Moreover, effect of PEO and crown ethers as phase transfer catalysts has been compared. In a low-polarity solvent, oligoethylene oxides are more effective catalysts, while in a polar solvent (butanol) the effectiveness of PEO and crown ethers as phase transfer catalysts is similar. 

Polymeric sulfones obtained by radical copolymerization of monomeric sulfones with styrene are active interfacial catalysts. Their activity was studied in the reaction of n-CgH]7Br with MI (M = Li, Na, K) in a toluene-water system [191]. Reaction of n-CgHiyBr with Nal at 100 °C for 48 hours produced only traces of n-CigH]7l when catalysts, including such low-molecular weight catalysts as DMSO, methyl phenyl sulfoxide and methyl benzyl sulfoxide, were absent. When this reaction was catalyzed by polymeric sulfone, the yield of n-CigH,7l was 43%. However, in the presence of polymeric catalyst an 82% yield was obtained after a reaction time of 160h. 

The terminal R groups can be aromatic or aliphatic. Typically, they are derivatives of monohydric phenoHc compounds including phenol and alkylated phenols, eg, /-butylphenol. In iaterfacial polymerization, bisphenol A and a monofunctional terminator are dissolved in aqueous caustic. Methylene chloride containing a phase-transfer catalyst is added. The two-phase system is stirred and phosgene is added. The bisphenol A salt reacts with the phosgene at the interface of the two solutions and the polymer "grows" into the methylene chloride. The sodium chloride by-product enters the aqueous phase. Chain length is controlled by the amount of monohydric terminator. The methylene chloride—polymer solution is separated from the aqueous brine-laden by-products. The facile separation of a pure polymer solution is the key to the interfacial process. The methylene chloride solvent is removed, and the polymer is isolated in the form of pellets, powder, or slurries. 

Two complementai y reviews of this subject are by Shah et al. AIChE Journal, 28, 353-379 [1982]) and Deckwer (in de Lasa, ed.. Chemical Reactor Design andTechnology, Martinus Nijhoff, 1985, pp. 411-461). Useful comments are made by Doraiswamy and Sharma (Heterogeneous Reactions, Wiley, 1984). Charpentier (in Gianetto and Silveston, eds.. Multiphase Chemical Reactors, Hemisphere, 1986, pp. 104—151) emphasizes parameters of trickle bed and stirred tank reactors. Recommendations based on the literature are made for several design parameters namely, bubble diameter and velocity of rise, gas holdup, interfacial area, mass-transfer coefficients k a and /cl but not /cg, axial liquid-phase dispersion coefficient, and heat-transfer coefficient to the wall. The effect of vessel diameter on these parameters is insignificant when D > 0.15 m (0.49 ft), except for the dispersion coefficient. Application of these correlations is to (1) chlorination of toluene in the presence of FeCl,3 catalyst, (2) absorption of SO9 in aqueous potassium carbonate with arsenite catalyst, and (3) reaction of butene with sulfuric acid to butanol. 

Preparation of siloxane-carbonate segmented copolymers by interfacial polymerization involves the reaction of carboxypropyl-terminated siloxane oligomers with bisphenol-A and phosgene, in the presence of a strong base and a phase transfer catalyst, in water/methylene chloride solvent system l50 192), as shown in Reaction Scheme XIV. 

In some cases, the Q ions have such a low solubility in water that virtually all remain in the organic phase. ° In such cases, the exchange of ions (equilibrium 3) takes place across the interface. Still another mechanism the interfacial mechanism) can operate where OH extracts a proton from an organic substrate. In this mechanism, the OH ions remain in the aqueous phase and the substrate in the organic phase the deprotonation takes place at the interface. Thermal stability of the quaternary ammonium salt is a problem, limiting the use of some catalysts. The trialkylacyl ammonium halide 95 is thermally stable, however, even at high reaction temperatures." The use of molten quaternary ammonium salts as ionic reaction media for substitution reactions has also been reported. " " ... 

The recovery of petroleum from sandstone and the release of kerogen from oil shale and tar sands both depend strongly on the microstmcture and surface properties of these porous media. The interfacial properties of complex liquid agents—mixtures of polymers and surfactants—are critical to viscosity control in tertiary oil recovery and to the comminution of minerals and coal. The corrosion and wear of mechanical parts are influenced by the composition and stmcture of metal surfaces, as well as by the interaction of lubricants with these surfaces. Microstmcture and surface properties are vitally important to both the performance of electrodes in electrochemical processes and the effectiveness of catalysts. Advances in synthetic chemistry are opening the door to the design of zeolites and layered compounds with tightly specified properties to provide the desired catalytic activity and separation selectivity. 

The Arrhenius plots of the CO conversion rate in Fig. 2 indicate that the activation energy for the Au/Nano-Ti02 catalysts is nearly zero. Haruta et al. [6] also reported similar observations. They suggest that this occurs when the CO adsorbed on gold particles reacts with adsorbed O2 on the support at the interfacial junction between the two surfaces. 

The different classes of Ru-based catalysts, including crystalline Chevrel-phase chalcogenides, nanostructured Ru, and Ru-Se clusters, and also Ru-N chelate compounds (RuNj), have been reviewed recently by Lee and Popov [29] in terms of the activity and selectivity toward the four-electron oxygen reduction to water. The conclusion was drawn that selenium is a critical element controlling the catalytic properties of Ru clusters as it directly modifies the electronic structure of the catalytic reaction center and increases the resistance to electrochemical oxidation of interfacial Ru atoms in acidic environments. 

The second general method, IMPR, for the preparation of polymer supported metal catalysts is much less popular. In spite of this, microencapsulation of palladium in a polyurea matrix, generated by interfacial polymerization of isocyanate oligomers in the presence of palladium acetate [128], proved to be very effective in the production of the EnCat catalysts (Scheme 3). In this case, the formation of the polymer matrix implies only hydrolysis-condensation processes, and is therefore much more compatible with the presence of a transition metal compound. That is why palladium(II) survives the microencapsulation reaction... 

---